1. Field of the Invention
The invention relates in general to a method of manufacturing a dielectric film of a capacitor in a dynamic random access memory (DRAM), and more particularly to a method for improving the charge storage ability of the capacitor.
2. Description of the Related Art
Typically, a single DRAM cell comprises a metal-oxide-semiconductor (MOS) and a capacitor used to store signals therein. As more charges stored in the capacitor, the less the effect of the noise is when the data are read by a reading amplifier. Additionally, the refreshing frequency is reduced as the charges stored in the capacitor are increased. Currently, the method of increasing the charges storage ability includes: (1) increasing the surface of the capacitor to increase the amount of the charges stored in the capacitor, but this decreases the integration of the DRAM; (2) choosing a proper dielectric material to form a dielectric layer of the capacitor with a relatively high permittivity to increase the amount of the charges stored per unit surface area of the capacitor; and (3) decreasing the thickness of the dielectric layer.
Conventionally, the dielectric film of the capacitor is an oxide-nitride-oxide (ONO) dielectric layer, which is a sandwich structure of an oxide layer, a silicon nitride layer and an oxide layer. The method of forming the ONO dielectric layer comprises the steps of forming a native oxide layer on the lower electrode, which is the storage electrode, and forming a silicon nitride layer on the oxide layer. An oxide layer is formed on the silicon nitride layer. The native oxide layer is naturally formed and cannot be easily removed. The dielectric constant of the native oxide layer is lower than the dielectric contact of the nitride layer. The dielectric constant of the dielectric film of the capacitor thus cannot be decreased.